Composite translucent thermal solar collector

ABSTRACT

In a composite translucent thermal solar collector, an upper transparent alveolar slab ( 4 ), an inner alveolar slab ( 3 ) and a third slab ( 2 ) of foam or fibrous material adjusted behind said alveolar slab ( 3 ), are kept packed together by elastic means ( 5 ) in a way that it allows the expansion of each slab according to its temperature and its coefficient of expansion. The lower face ( 11 ) of the inner alveolar slab ( 3 ) is black opaque while the other face is clear transparent, in order to obtain the conversion of the visible fraction of the solar radiation in IR radiation in a inner region of the collector, under “greenhouse” conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of PCT Patent Application No. PCT/IT2006/000128 filed Mar. 3, 2006, which claims priority of Italian Patent Application No. TO2005A000141 filed Mar. 4, 2005.

FIELD OF THE INVENTION

The present invention attains to the exploitation of the solar energy. The solar energy may be exploited either to produce electricity (photovoltaic effect), either to heat water (or another liquid) (thermal effect), which is the field of the present invention.

BACKGROUND OF THE INVENTION

Actually various kind of thermal solar collectors are commercialised and they differ for secondary aspects. Substantially they are all constituted by black pipes into which a liquid, having an elevated boiling point and low freezing point, circulates, said liquid having function of heat exchanger. The differences concern usually the geometry of the collector and in case of the pipe, the nature of the materials forming the collector and the ways of insulation of the collector. The efficiency of these systems is rather low, so that they are coupled with other heating systems (usually an electric coil) which allow to overcome periods of low solar irradiation (mainly due to cloudiness).

The reason of the limited capacity of energy collection depends mainly on two structural elements:

a) the fact that the collector is opaque, even if it is black, involves important heat losses, because the conversion of the wavelength (visible™ infrared (IR)) takes place on the external surface of the collector, while the convector liquid circulates inside the pipe. This generates parasitic losses by reflection and irradiation by the black pipe.

b) The use of a heat exchange liquid generates further lowering of efficiency.

In order to improve the efficiency, valuable materials are used in the fabrication of the panels: this increases the costs of the solar panels thus limiting their diffusion. It should be here remembered that on the Earth surface the falling solar energy (W) is constant: the only method to increase the collected energy is to increase the collector's surface. This solution is penalised by the high costs of the solar panels which are currently commercialised. The first drawback is economic and the second one is technical and both contribute to limit the solar collectors' diffusion.

SUMMARY OF THE INVENTION

The aim of present invention is to act on both drawbacks in order to improve and to ease the exploitation of the solar energy. The solar collector devised by this invention perfectly achieves this aim by realising the visible wavelengths conversion (visible ™IR) internally, in intimate contact with the liquid to be heated. This effect is achieved with special efficiency because of the presence of the following combination of elements:

1) an inner alveolar polypropylene slab (or other similar material) as those usually commercialised, comprising two parallel walls kept separated by equidistant baffles. The two walls and the baffles form, together, a series of tubular alveoi. These slabs should be produced of material as much transparent as possible. One of the two walls has to be coextruded using additives to make it as black as possible. The result is a sheet composed of a wall perfectly translucent and the other perfectly opaque black. Within the alveoli of the slab the water is kept circulating. In order to achieve this, two headers are applied to the two open ends of the slabs. These headers are tubular (larger than the thickness of the slab) and open laterally on the length by which they are slipping over the ends of the slab. Once these headers are closed on the ends of the slab, they are either glued by an appropriate glue or weld together with the slab by heat. The resulting structure is illustrated in FIG. 2. The two terminals of this newly formed pipes form, with the alveoli to which they are perpendicular, a network of pipes. The headers are closed at their ends and in two (out of four) a pipe-fitting structure is applied.

2) an upper alveolar slab, placed before the inner slab, and made of polycarbonate (with the external wall protected against degradation produced by UV radiation) or of a more resistant, but more expensive PMMA (Polymethylmethacrylate=Plexiglas), or of other similar materials, with the alveoli sealed in order to form a forward thermal insulating air cushion. Characteristic of this slab is to be perfectly transparent.

3) a third slab of foam or fibrous material, like polystyrene, polyurethane or rock wool, arranged on the back side of said inner transparent slab in order to ensure its insulation.

4) a C or U shaped open case keeps the slabs tied together and attached to a flat frame which supports the whole structure, keeping the three slides in close contact, but allowing them to expand independently according their own specific coefficient, without generating a bending in the whole set as it happens if the various parts described above are built as a unitary set.

5) an axle rotating vertically and driven either manually or by a motor (eventually controlled electronically) is attached to the frame in order to seasonably orientate the whole set.

Various types of solar panels have been disclosed in the patent literature. For instance, U.S. Pat. No. 4,114,597 discloses a solar panel which is a one-piece synthetic thermoplastic unit in which certain portions are solar energy collecting and other portions are transparent or translucent to solar energy.

The solar panel object of this invention has some similarities with this system, but differs from it in very fundamental aspects which have functional grounds.

The first and fundamental difference is that the U.S. Pat. No. 4,114,597 patent is, as it recites in the title, a unitary solar collector, while the solar panel according to the present invention is a composite solar collector, constituted of two different alveolar slabs, one of which is of polycarbonate co-extruded with an UV protecting layer like those devised by European Patent 110238 or EP 0283072, or any other else or preferably of PMMA which withstands better without addition the action of UV radiation. The second slab is similar, but it differs from the first one, being of transparent polypropylene co-extruded only with black polypropylene, so that one wall, and not the baffles as in patent U.S. Pat. No. 4,114,597, is black and opaque, capable of absorbing solar radiant energy. The ground of this difference is that a “unitary” panel comprising an alveolar clear region and a black region would be submitted to differential thermal expansions of the two “regions”, which in turn would produce curling of the slab, curling which would be conspicuous if the panel is rather long, as devised in U.S. Pat. No. 4,114,597.

Another fundamental difference from U.S. Pat. No. 4,114,597, which does not require an external frame, is that the present invention provides a supporting frame and a tubular elastic case to connect together the two transparent slabs and the insulating foam slab in order to avoid the curling deriving from the different thermal expansion of the two transparent slabs. Furthermore, the first and second regions mentioned in the above U.S. Pat. No. 4,114,597, are, in the present invention, separated by a physical discontinuity. This discontinuity, which is used to form an additional thermal insulating space, is devised in order to avoid friction between the two slabs during the thermal expansion, which is different in the two slabs. The tubular elastic case closes together the supporting frame, the insulating foam slab, the polypropylene and the forward transparent slabs. In this way, the three slabs are free to expand themselves according to their respective temperatures and their specific thermal coefficients, without being bound to bodies subject to different expansion. This contrivance of the system allows the free thermal expansion of each element: not being bound one to another they expand flatly without disturbing curling.

Polycarbonate is known to be degraded and damaged by the UV wavelengths of the solar light. Consequently the external slab should be protected from UV on its outer wall.

Various systems are used to protect the slab of polycarbonate. European patent No. 110238 describes one method and European patent EP 0283072 describes another procedure. The alternative is to use a different material more resistant to UW radiation like PMMA or any other apt material.

The solution of the present invention also shows, only apparently, some similarities with the structures disclosed by Patent WO03085329 which comprises “extruded translucent thermoplastic sheets”, but it requires “a dark heat absorbing fluid by solar radiation”, which works as heat exchanger fluid. It does not suggest the blackening of the internal side of the back of the wall for the conversion of the visible→IR, but the blackening is considered optional and it is not a basic feature for heat absorption/transformation and it is optionally applied as supplement of insulation to protect the underlying structure. It is a very peculiar idea completely different from the mechanism of the present invention. Furthermore, it should be stressed that in this cited patent, because of the dark colour of the fluid, the heat absorption takes place only on the surface of the liquid and hardly penetrates it and reaches the blackened back wall. In conclusion the system devised by this cited patent differs for various aspects from the solution of the present invention and its efficiency is lower. Furthermore, the cited patent requires the interposition of a heat exchanger between the heat collection and the heated fluid with consequent losses of efficiency. Moreover, when it considers the possibility of using a double alveolar sheet, it does not provide any indication how to prevent distortions and curling due to the differential thermal expansion of the two sides of the sheet, one full of black fluid and the other one empty and simply translucent. A further difference is that it discloses “a solar radiation capturing layer above the top layer” i.e. it provides superimposing a black heat absorbing layer thus making the conversion of the solar radiation (visible→IR) external to the system. Another only apparent similarity is that the same US patent states that the bottom layer may be black or blackened with various systems varying from co-extrusion to painting and to application of black lining or film which have the purpose of protecting the underlying roof structure. However, in the specification it is admitted that the sunlight hardly reaches the bottom of the channel, because the black liquid, circulating in it, shadows the black wall. As a conclusion the differences appear to be fundamental and technically relevant.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is hereafter described in detail with reference to the attached figures wherein:

FIG. 1 is a view in cross section of the solar collector according to the invention.

FIG. 2 is a view in longitudinal section of a single alveolus modified according to the invention.

FIG. 2 bis is a view of an alternative header.

FIG. 3 is a view, corresponding to FIG. 1, of the solar collector according to the invention, into which the systems of support and orientation of the collector are outlined.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, number 1 indicates a frame, which could be continuous or reticular, which fulfils the double function of supporting the collector and of connecting it to the axle 19 of FIG. 3 which will be explained later on. Reference 2 indicates the profile of the insulating back panel, constituted of foam polystyrene or polyurethane or conveniently of any other foamy, or spongy or fibrous material, which may be a good thermal insulator. Number 3 indicates an alveolar polypropylene which constitutes the care of the solar collector. Number 4 indicates the other slab made of transparent alveolar polycarbonate or Plexiglas (PMMA) which is arranged over the slab 3. Reference 5 indicates the elastic case which keeps together the various components of the thermal solar collector according to the invention. The alveoli 6 of the forward slab are separated by ribs 7. Number 8 indicates the ribs of the inner slab 3. The back wall 9 of the slab 4 faces the forward wall 10 of the slab 3; the black back wall of the slab 3 is indicated by 11. Number 12 indicates an alveolus of the inner slab 3 while 13 indicates the forward wall of the slab 4.

In FIG. 2 it is clearly shown the header 15 closed on the sides with sealing elements 16 and the black wall 11 resulting by the co-extrusion of the slab 3. The headers 15 are applied water tightly at the ends 17 of the alveolar slab 3. Two out of the four sealing structures 16 bear a pipe fitting 18 to connect the headers and the pipes externally to the panel.

FIG. 2 bis represents an alternative header structure. In this case the header is a channel which is not constituted by an applied pipe, but it is obtained by milling the ribs and one wall, along a limited portion of the alveolar slab, thus bringing the alveoli of the slab in reciprocal communication. The channel thus opened is made water tight, by applying by gluing or welding of a thin flat element of compatible material. The circulation of the liquid inside the alveolar slab, thus modified, is obtained by the application, by gluing or welding of pipe fitting structures to the above cited flat element.

The water tightness of the alveolar slab, modified as above indicated, is completed by melting together the two walls of the alveolar slab and the interposed ribs into a single body which seals the ends of the alveoli.

As it can be seen, in FIG. 2 bis, number 33 is a pipe-fitting structure, 34 shows the sealing of the ends of the alveoli, in order to avoid loss of water. Reference 36 shows the cavity excavated by milling the ribs, thus forming a channel that puts in contact all the alveoli, thus allowing the water circulation into the slab. Number 35 is a flat cover of such newly excavated channel, which transforms it into a head-pipe 36. With reference 37 the ribs siding the alveoli are indicated and with reference 38 the two walls of the slab are indicated.

In FIG. 3 it is clearly visible the cross section of the axle 19 rotating the solar collector by the handle 20. Number 21 indicates the stand supporting the axle 19 and the frame 1 that supports the entire solar collector. The reference 22 is used to adjust the inclination of the solar collector: it has imprinted notches labelled either with angular degrees or with letters which are brought against the reference 23. The letters may be the initials of the months in order to optimise the inclination of the solar panel with respect to the incidence angle of the solar rays.

Hereafter the explanation of figures goes on with more details.

The slab 3 is co-extruded with charcoal powder or other additives in order to make one of the walls perfectly black and opaque, precisely wall 11 which will be positioned on the back side in respect to the path of the light. The ribs which side the alveoli and the forward wall, on the contrary, should be perfectly clear and transparent. The alveoli 12 house the water (or any heat transfer liquid) which is heated by the sun. As the manufacturing process of the slabs does not provide the closure of the alveoli the solutions of FIGS. 2 and 2 bis have been provided in order to make the alveoli as tight chambers into which the water can be circulated.

As previously mentioned, number 17 indicates the hedge of the alveolus of a slab produced commercially, while number 15 indicates the header according to this invention. It is open on its length side in which it terminates with two flanges which are used to weld or to paste the header to the slab. The two open ends of the pipe 15 are plugged by the plug 16 which is suitably pasted or welded on the edges of the pipe 15. These are necessarily four per each slab and two bear a pipe fitting structure 18. The preferable position of these two structures is diagonal so that the water stream may cross the whole panel without creating preferential courses that exclude parts of the panel.

With reference again to FIG. 1, the slab 4 of polycarbonate or of Plexiglas (PMMA), is applied on the forward side of the panel: it has the function of thermally insulating the slab 3 where the whole process takes place, avoiding losses of heat from the water contained therein. Its function is similar to that of the slab 2 of polyurethane, but being on the way of the light it should be perfectly clear and transparent to the visible fraction of the solar radiation. A slab of polycarbonate or of Plexiglas satisfies perfectly these requirements. The hedges of the alveoli 6 are suitably sealed in order to avoid movements of air from outside to inside and vice versa with losses of heat from the alveoli. The forward wall 13 and the back wall 9 and the ribs 7 should be as clear and transparent as possible. The alveoli 6 shown in cross section in FIG. 1, suitably contain air.

If polycarbonate is used, it should be of the type with UV protection. PMMA is not sensitive to UV radiation, but is more expensive.

Before going in further details of the preferred embodiment of this invention, shown in FIG. 3, it is necessary to hint at the operation of a solar collector according this invention, as above described, and at its differences from the solar collectors actually on the market.

Preliminarily some details on solar radiation arriving on the Earth are necessary. The solar radiation arriving on the Earth comprises many wavelengths between 250 and 3000 nanometers, each with its own characteristics. The wavelengths between (approximately) 420 and 670 nanometers constitute the visible band (i.e. that is perceived by the human eye). The colour of a body becomes apparent because this, hit by the solar radiation, absorbs all the wavelengths with the exception of one (the “colour” indeed) that is reflected and consequently perceived by the eye of the observer. A white body appears so because it reflects all the visible wavelengths, on the opposite a black body appears so because it absorbs all the wavelengths that hit it and no one is reflected nor any colour is visible. However the solar radiation absorption is not free of consequences for the body hit: this latter interacts with the solar radiation being heated. Increasing its temperature a black body tends, in turn, to emit warn radiation toward the environment. This radiation is characterised by wavelengths comprised in the range of the infrared (IR), i.e. between the 670 and the 3000 nanometers. The glass, the polyethylene, the Plexiglas, the polycarbonate and other similar substances are permeable to the visible radiations, but they stop the passage of IR radiations. If, in a region closed by one of these substances, a body, more or less dark, is hit by the visible fraction of the solar radiation, the “greenhouse effect” is generated, i.e. the heat generated by the dark body cannot be dispersed in the open environment and consequently it increases and it is accumulated within the closed region.

The classical solar collectors, currently on the market, are essentially constituted by black pipes, which, hit on their external surface by the solar light, are heated. The solar technology is hinged on the quality of the materials composing the black pipe. Certain metals convey better than others the heat into the pipe to the liquid circulating, which is better heated. But the pipe being a black body emits heat, especially toward outside. For this reason the black pipes are housed in a chamber closed by transparent material (glass, polycarbonate and similar materials) in order to obtain around the pipes a “greenhouse environment” with the purpose of limiting the losses of heat, facilitating its conveyance toward the liquid circulating inside the pipes. Other manufacturers follow a different course: instead of using artefacts very efficient, but very expensive they follow a different course, using black plastic substances (e.g. polypropylene) which together with a lower efficiency have much lower costs, allowing to expose to the solar radiation much wider surfaces, having costs lower than the metal types.

The rationale of this invention is, on the opposite, totally different: the black surface, which absorbs the solar radiation, is not in the open air, nor external to the pipe, but is internal to the alveolus, region characterized by a “greenhouse condition”. The polycarbonate, the polypropylene, the Plexiglas and similar materials are good thermal insulating materials. Consequently the heat which is formed by this process on the inner surface of the alveolus hardly finds its way outside of the alveolus 12 into which water to be heated is circulated. Two profitable conditions are thus achieved: the water perfectly contacts all parts from which it should extract heat. Moreover, because of its high specific heat, water is particularly apt to store the heat.

Although not illustrated in the drawings, the presence of a pump to circulate the water is implied. With a suitable design it is possible to exploit the thermosiphon effect.

It is now possible to examine the preferred embodiment of this invention illustrated in FIG. 3. It should be recalled that, complementary to the above illustrated features, there are the orientation and the inclination of the solar panel in order to optimise the exploitation of the solar energy. It is well known that the following equation assesses the real amount of energy falling on the Earth surface E=sen α*W, where W is the total energy that arrives from the Sun and α is the angle of incidence. Unfortunately the angle α changes continuously because of two concurrent factors: the apparent solar movement from the site of sunrise to that of sunset and the seasonal variation of altitude (above the horizon). To solve the latter problem, the embodiment illustrated in FIG. 3 appears perfectly suitable. The frame of the panel illustrated in FIG. 1 is joint integrally to the rotating axle 19. This is kept conveniently removed above the soil/floor by the stand 21 at height sufficient to allow the rotation of the frame 1 with what it bears. The chance of inclination may be performed either manually, operating the handle 20 or it may be conveniently performed thanks to a worm gear which can be operated either manually or by a motor optionally controlled electronically.

The adjustment conveniently monthly or bi-weekly, aims to make at noon the incidence angle=90° in order to obtain a value sen α=1. To make this adjustment easier index 22 is conveniently fastened to the rotating axle 19. On the index it may be convenient to indicate the initials of the months which should be brought to coincide with the fix index 23, according to the seasonal evolution.

According to a preferred embodiment of the invention, the thermal solar collector is associated to a pool-reservoir to store the warm water. The dimensions of the pool are proportional to the surface of the solar panel. It is fundamental that the water's surface is sheltered from any contact with the air of the environment in order to prevent any evaporation from the water mass which would subtract huge amount of calories from the same. This aim can be achieved by laying down on the water surface a plastic film which should be kept flowing on the water's surface. 

1. A thermal solar collector for conversion of visible fraction of the solar radiation in infrared radiation, useful for heating, comprising: a first forward heat insulating slab (4), an inner heat insulating slab (2) and an inner, lower and double colored alveolar slab (3); said first forward alveolar heat insulating slab (4) provides a forward thermal insulation to said inner lower and double colored alveolar slab (3); said inner heat insulating slab (2) adjusted behind said inner, lower and double colored alveolar slab (3); said inner, lower and double colored alveolar slab (3) comprising: a second clear and transparent forward face; a black opaque internal face (11) said black opaque internal face is between said second clear and transparent forward and said inner heat insulating slab; and slab headers (15), located at the end of alveoli, and adapted to let water to be heated to circulate inside along said inner, lower and double colored alveolar slab and enter said alveoli (12); wherein conversion of visible fraction of the solar radiation in infrared radiation takes place on said black opaque internal face (11), said conversion being under greenhouse conditions such that the heat generated by said conversion of visible fraction of the solar radiation into infrared radiation by said black opaque internal face (11) is internal to one of said alveoli; said water to be heated in contact with said black internal face (11).
 2. The thermal solar collector of claim 1, wherein said first forward alveolar heat insulating slab (4) is made of material clear and transparent to the visible fraction of the solar radiation with said alveoli closed in order to avoid leakage of the inner air.
 3. The thermal solar collector of claim 1, wherein said inner heat insulating slab (2) made of foam or fibrous material and provides back thermal insulation to said inner, lower and double colored alveolar slab (3).
 4. The thermal solar collector of claim 1, further comprising an elastic windowed case (5) which keeps tied together said first forward alveolar heat insulating slab (4), said inner heat insulating slab (2), said inner, lower and double colored alveolar slab, and a back frame (1) while allowing the expansion of each slab according to a temperature and a coefficient of expansion, without creating curling and bending in the whole panel.
 5. The thermal solar collector of claim 1, further comprising a vertically rotating axle (19) operated either manually or by motor on said axle being fastened said frame (1) which in this way can be conveniently oriented.
 6. The thermal solar collector of claim 1, further comprising separating ribs in at least one of said first forward heat insulating slab (4), said inner heat insulating slab (2), and said inner, lower and double colored alveolar slab.
 7. The thermal solar collector according to claim 1, wherein said first forward alveolar heat insulating slab is made of polypropylene, polycarbonate, or polymethylmethacrylate.
 8. The thermal solar collector according to claim 1, wherein said first forward alveolar heat insulating slab is made of polymethylmethacrylate or of UV protected polycarbonate.
 9. The thermal solar collector according to claim 1, wherein said slab headers (15) are water tight and pipes are applied at the end (17) of the alveoli and are closed on the sides with sealing elements (16).
 10. The thermal solar collector according to claim 6, wherein every one of said slab headers (15) is a channel (36) obtained by milling the ribs and one wall, along a limited portion of said inner, lower and double colored alveolar slab, thus bringing the alveoli of said inner, lower and double colored alveolar slab in reciprocal communication, said channel thus opened being made water tight, applying by gluing or welding a thin fat element (35) of compatible material.
 11. The thermal solar collector according to claim 10, wherein the circulation of the liquid inside said inner, lower and double colored alveolar slab is obtained by gluing or welding, a pipe fitting structure (33) to the flat element (35).
 12. The thermal solar collector according to claim 1, further comprising a pool-reservoir to store warm water; and a plastic film laid on the surface of said pool reservoir. 13 The thermal solar collector according to claim 5 further comprising an electronic controller for said motor.
 14. The thermal solar collector according to claim 2, further comprising a pool-reservoir to store warm water; and a plastic film laid on the surface of said pool reservoir.
 15. The thermal solar collector according to claim 3, further comprising a pool-reservoir to store warm water; and a plastic film laid on the surface of said pool reservoir.
 16. The thermal solar collector according to claim 4, further comprising a pool-reservoir to store warm water; and a plastic film laid on the surface of said pool reservoir.
 17. The thermal solar collector according to claim 5, further comprising a pool-reservoir to store warm water; and a plastic film laid on the surface of said pool reservoir.
 18. The thermal solar collector according to claim 6, further comprising a pool-reservoir to store warm water; and a plastic film laid on the surface of said pool reservoir. 